1. Field of the Invention
The field of the invention is that of acoustic wave devices and specially that of filters or resonators made on the surface of a substrate made of piezoelectric material.
The material used for making a surface acoustic wave device and the cut of the material are generally chosen on the basis of two criteria: the piezoelectric coupling coefficient that characterizes the maximum relative band that can be obtained and the behavior of the material as a function of the temperature. Indeed, when the temperature rises, the material expands and its coefficients of elasticity vary, giving rise to variations in the speed of propagation of the surface wave and hence in the frequency of the devices.
2. Description of the Prior Art
Quartz is a material that has long been used for surface acoustic wave filters. The cut generally used is what is called the ST cut (M B. Schultz, B. J. Matzinger, M. G. Holland, "Temperature Dependence of Surface Acoustic Wave Velocity on .alpha. Quartz", Journal of Applied Physics, Vol. 41, No. 7, pp. 2755-2765, (1970)). The cutting angles .phi., .theta. and .psi. are defined by the IEEE standard (Standard on Piezoelectricity Std 176-1949, Proc. IRE, Vol. 37, pp. 1378-1395 (1949)).
To avoid ambiguity in the definition of the cutting angles, we shall recall the definition of these angles here below. The axes X, Y and Z are the crystallographic axes of the crystal. The crystal chosen is known as a "left-hand quartz" crystal and is characterized by the sign of the piezoelectric constants e.sub.11 and e.sub.14. "Left-hand quartz" is quartz such that e.sub.11 is positive and e.sub.14 is negative. FIG. 1 shows a plate with a cut (XY). This means that the normal to the cutting plane is the axis Y and the direction of propagation is the axis X. This figure defines the direction W (along the width of the plate), I (along its length which is therefore the direction of propagation) and t (perpendicular to the plate). A cut and an angle of propagation are defined starting from the cut (YX) and by applying three successive rotations. The rotation about W is a rotation by an angle .phi., the rotation about I by an angle .theta. and the rotation about t by an angle .psi.. The propagation takes place along the direction I of the rotated plate. The cutting plane is therefore defined entirely by the two angles .phi. and .theta. (FIG. 2a) while the third angle .psi. defines a particular direction in this plane and therefore, for the surface waves, the direction of propagation used (FIG. 2b).
Here below, we shall recall the angles defining different cuts commonly used for volume wave filters:
BT cut: .phi.=0, .theta.=-49.degree. PA1 AT cut: .phi.=0, .theta.=+35.degree. PA1 SC cut: .phi.=-22.4.degree., .theta.=+33.8.degree. PA1 a quartz substrate having a surface of propagation of surface acoustic waves; PA1 means to create transduction centers and reflection centers on said substrate; PA1 60.degree..ltoreq..phi..ltoreq.0.degree. PA1 .theta. is contained in a range of .+-.40.degree. around -40.degree..cos(3.phi.) PA1 .psi. is contained in a range of .+-.22.5.degree. around 35.degree.+10.degree..sine(3.phi.) PA1 if CTF.sub.1 : first-order coefficient PA1 CTF.sub.2 : second-order coefficient. PA1 which corresponds to a temperature of reversal ##EQU1## PA1 the following temperature of reversal is obtained T=To-10.degree. K
The ST cut commonly used for surface wave devices is defined by the angles .phi.=0.degree., .theta.=42.75.degree. and .psi.=0.degree.. This cut is described in FIG. 3. The initial axial reference system is (XYZ). After the first rotation it becomes (X'Y'Z'), after the second rotation (X"Y"Z"), and after the third rotation (X'"Y'"Z'").
This single-rotation cut has the advantage, for the Rayleigh waves, of showing a variation of the frequency with the temperature that is parabolic with a reversal point at about 25 degrees Celsius (25.degree. C.), i.e. often in the middle of the range of temperature of operation for the filters. The coupling coefficient for this cut is relatively low (about 0.12%).
If we deposit a material such as aluminum for example, especially to make a transducer that generates surface acoustic waves on the surface of the substrate, the reversal temperature point changes. Indeed, to compensate for this effect and keep a reversal temperature point close to 25.degree. C., the cut is slightly changed. This leads to the use of cuts starting from .theta.=30.degree. up to .theta.=42.degree.. These cuts all have characteristics very close to each other, apart from the speed of propagation of the waves and the reversal point of their frequency-temperature relationship.
In the past few years, increasing numbers of filters have been made with low insertion losses. To reduce insertion losses, the reflection of surface waves on electrodes is often used. Most frequently, the material used for the electrodes is aluminum. The filters that use these reflections are, for example, resonator filters or one-directional filters of the DART or SPUDT type. The coefficient of reflection on an electrode is a very important characteristic for the designing of these devices. Indeed, the greater the coefficient, the easier will it be to achieve low losses with a small chip size. Furthermore, there are also surface acoustic wave resonators being made that use other types of reflectors such as etched gratings or, more often, gratings of metallized lines to form cavities. For this type of device also, the reflection coefficient is an important characteristic as a high reflection coefficient makes it possible to have a cavity with lower losses for the same size and therefore improves the Quality factor of the resonator.
It must be noted however that, when a device is being designed, the reflection coefficient is limited by the maximum thickness of the electrodes that can be made. Furthermore, the sensitivity to the technological uncertainties of manufacture (in terms of the thickness of the metallization or the line width) is all the greater as the thickness of the metallization is great. This limits the thickness that can be used in practice.